We have observed a lack of rapid expansion of naloxone receptor binding in previously morphinized post-addict rats after an acute morphine injection which we interpret as the development of lasting tolerance to opiates. Opiate stereospecific receptor binding technic will be used 1) as a physical chemical probe of this type of tolerance; 2) as a method of determining a relationship between such tolerance and relapse to morphine-induced drug-seeking behavior; 3) to determine whether compounds, e.g. alpha-methyl-p-tyrosine, haloperidol, etc. shown to modify morphine-induced drug seeking behavior also modify opiate receptor binding; and 4) to determine whether the persistence of receptor tolerance parallels the duration of morphine depressed RNA synthesis of rat brain nuclei.